The present invention relates in general to automotive engines and, particularly, to a multiple-cylinder reciprocating-piston spark-ignition internal combustion engine with exhaust gas cleaning arrangements. The present invention further relates to a method of reducing the concentrations of toxic compounds in exhaust gases of an internal combustion engine of the specific type.
A variety of attempts have thus far been made so as to reduce the concentrations of toxic compounds such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x) in the exhaust gases emitted from automotive engines. The most basic and most important consideration to be paid in realizing such attempts on a practical basis is doubtlessly to preclude production of the toxic compounds in the process of combustion of air-fuel mixture in the combustion chambers of an engine. There have however been limitations in preventing production of toxic compounds by recourse to improvement in burning conditions of the air-fuel mixture in the combustion chambers of an engine. It has therefore been proposed and put into practice to have the exhaust systems of internal combustion engines equipped with after-burning devices such as thermal reactors and oxidative catalysts for the purpose of re-oxidizing unburned residues contained in the exhaust gases emitted from the combustion chambers of the engines. Such devices are thus successful in eliminating hydrocarbons and carbon monoxide in the exhaust gases but are not effective to process nitrogen oxides in the exhaust gases. Catalytic devices operative to reduce nitrogen oxides into harmless compounds are known but are not fully acceptable because of the lack of reliability, low cost-performance and insufficient durability of the catalysts used in such devices. Controlling the formation of nitrogen oxides in the combustion chambers of an internal combustion engine has therefore been decisive in cleaning up the exhaust gases from the engine. One of the useful expedients to preclude production of nitrogen oxides in the combustion chambers of an internal combustion engine is, as is well known in the art, to have the exhaust gases recirculated into the mixture supply system of the engine at a controlled rate so as to lower the maximum burning temperatures to be reached in the combustion chambers to such an extent as will reduce the concentration of nitrogen oxides to an acceptable level. In view of the fact that the recirculated exhaust gases per se are not contributive to the combustion of the fuel in the mixture, the concentration of nitrogen oxides produced in the combustion chambers of an engine must be reduced by increasing not only the proportion of the recirculated exhaust gases but the proportions of other chemical compounds not contributive to the combustion of the mixture in the combustion chambers to the fuel content of the mixture supplied to the engine cylinders. The compounds not contributing to the combustion of the mixture in the combustion chambers may include, in addition to the recirculated exhaust gases, carbon dioxide (CO.sub.2) and water vapor (H.sub.2 O) contained in the residual exhaust gases which have failed to be discharged from the combustion chambers during the exhaust stroke of the preceding cycle of operation and thus remaining in the combustion chambers, and nitrogen (N.sub.2) inherently contained in the air-fuel mixture supplied to the engine cylinders. If the mixture is leaned off for the purpose of reducing the concentrations of hydrocarbons and carbon monoxide, the gases not contributing to the combustion of the mixture will further include oxygen (O.sub.2) contained in the excess air in the mixture. Such compounds not contributive to the combustion reaction in a combustion chamber will be hereinafter referred to as "inactive compounds" or "inactive gases" contained in the "mixture gases" produced in the combustion chamber and containing the air-fuel mixture inducted into the combustion chamber from a mixture supply system such as a carburetor of an internal combustion engine. Furthermore, the "leaned" air-fuel mixture as herein referred to means an air-fuel mixture which is leaner than a stoichiometric mixture and which is thus proportioned to have a theoretical air-to-fuel ratio of approximately 14.7:1 in the case of an air-gasoline mixture.
Increasing the proportion of the inactive compounds to the fuel content in the mixture gases produced in a combustion chamber is conducive to reduction of nitrogen oxides in the combustion chamber but results in a slower combustion that tends to reduce the thermal efficiency of the engine especially when the engine is operating at high speeds. This gives rise to deterioration of the efficiency at which the heat energy generated in the combustion chamber is converted into mechanical power output of the engine and to an increase in the fuel consumption rate and, for this reason, there has been a limit to which the quantities of nitrogen oxides can be reduced by admixing desired quantities of inactive gases to the air-fuel mixture inducted into the combustion chamber. Furthermore, the reduction of the maximum combustion temperature as invited by the recirculation of the exhaust gases into the engine cylinders is responsible for incomplete combustion of the air-fuel mixture and results in increases in the concentrations of hydrocarbons and carbon monoxide in the exhaust gases emitted from the combustion chambers of the engine. The present invention contemplates elimination of these problems encountered in an internal combustion engine arranged to reduce the concentration of nitrogen oxides in the exhaust gases by increasing the proportion of the total quantity of the inactive compounds in the mixture gases produced in engine cylinders to the quantity of the combustible charges in the mixture gases.